The present invention relates generally to a system and method for laser light amplification and, more particularly, to a system and method for providing amplification of a laser beam emitted from a solid state laser that provides higher pulse rates, improved pointing stability, and optionally variable pulse rates for a variety of uses, including for non-destructive laser ultrasonic testing of materials.
Amplification of laser light is required for a variety of applications. Long haul telecommunication applications, such as those employing single mode optical fiber, often require optical repeater/amplifiers to boost sagging signal levels. Material processing applications may require very high power laser light to perform functions such as cutting of various materials and preparation of material surfaces. Applications requiring intense energy pulses of laser light employ some configuration for providing either time-varying optical amplification or intensity modulation of laser light.
One method for amplifying a laser beam is to employ a laser medium whose optical gain may be controlled by optical pumping. Optical pumping of a solid state laser medium is a common and conventional method used to create a population inversion of energy states for laser applications requiring high-gain. The laser medium providing high-gain, when optically pumped, may comprise a material such as neodymium yttrium-aluminum garnet (Nd3+:YAG), neodymium glass (Nd3+:glass), Erbium doped optical fiber (Er3+:silica), or Ruby rods (Cr3+:Al2O3). These materials are merely exemplary candidates for high-gain laser media, and those skilled in the art will appreciate that any suitable material capable of maintaining an inverted population of energy states when optically-pumped may serve as an optical amplifier. Those laser media utilizing Nd3+:YAG are common, given the substantial optical gain near desired wavelengths near the 1.064 xcexcm range. Additionally, Nd3+:YAG laser media provide linearity of pumping rate with respect to inverted population given its four-level transition system that is also well known to those skilled in the art.
To saturate an entire laser medium with an inverted population through optical pumping, a conventional method is to distribute a large array of laser diodes across the surface of the laser medium to form a pumping array. The light emitted from the individual laser diodes of the pumping array excites the laser medium and provide a very high optical gain for the energy transition level of the optically-pumped, inverted population within the high-gain laser medium, e.g., near the 1.064 xcexcm range for Nd3+:YAG, near the 1.06 xcexcm range for Nd3+:glass, near the 0.6943 xcexcm range for Cr3+:Al2O3, near the 1.55 xcexcm range for Er3+:silica, etc.
An integrated approach to performing laser light amplification and generating optical pulses utilizes gain switching of a laser medium. In this method of providing a high energy pulsed laser beam, the optical pumping of a high-gain laser medium itself is pulsed to generate a time varying gain of the high-gain laser medium through which a laser beam is propagating. This results in a pulsed output laser beam after an original laser light source has traveled through the high-gain laser medium that is being optically pumped in a time varying manner.
Each optical pumping cycle takes the high-gain laser medium through a transition which consists essentially of generating a sufficient energy state population through optical pumping to reach threshold for amplification. Before the optical pumping begins, the population of energy states is initially below threshold and optical amplification does not occur. After the high-gain laser medium has operated in an amplification mode for some time, then optical pumping is switched off, and the energy state population is subsequently depleted. By turning off the optical pumping, the population falls below threshold and the optical amplification is interrupted until the optical pumping again resumes and the population of energy states again reaches threshold. Such a method provides for a pulsing of the conditions in which laser light amplification may occur. Such a method is preferable to a method which merely blocks a highly amplified laser beam in that design considerations need not include the potentially loss energy due to the dumping of electromagnetic energy into a shutter assembly. Many other advantages are inherent to the fact that the solution is electronic, not incorporating any mechanical components for a mechanical shutter system.
Another method for providing an electronic solution is to generate a high energy pulsed laser beam to maintain continuous optical pumping of the high-gain laser medium and to modulate the high-gain laser medium""s loss coefficient. One method to perform such loss switching is to electronically modulate an optical absorber that is placed within the optical resonator cavity next to the high-gain laser medium. Such a configuration will permit the user to control the loss of the laser light traveling through the high-gain laser medium as opposed to controlling the rate at which optical pumping occurs. Those skilled in the art will recognize a variety of methods for performing loss switching of laser light contained within a the highgain laser medium including electrical modulation of an electro-optic crystal to perform intensity modulation of the laser beam.
Such a method is an extension of the gain switching method as an optical resonator""s threshold energy state population difference is proportional to the resonator""s loss coefficient. In this method, the loss coefficient is modulated to provide intermittent periods when the optical loss of the high-gain laser medium is prohibitively high to maintain oscillation. This results is creating an increased energy state threshold population to sustain oscillation, given the increased loss of the high-gain laser medium. Even though the energy state population would be sufficiently high for oscillation were the loss coefficient of the high-gain laser medium not increased, no optical amplification can occur during the period when the loss coefficient is elevated.
When the loss in suddenly decreased during the transition of a pulsing cycle the loss coefficient, the energy state population begins to deplete resulting from the decreased optical losses. The high-gain laser medium will amplify the laser light during the period when the energy state population exceeds the threshold condition for oscillation during the period that the loss coefficient is minimum. However, as the population continues to decrease, the population will eventually fall below the newly established energy state threshold for oscillation corresponding to the period of time when the loss coefficient is at its minimum during a modulation cycle.
These methods of performing electronic switching of either the gain or loss coefficients of the highgain laser medium often employ flashlamp optical pumping. The use of such a light source for performing the optical pumping presents some undesirable effects which significantly limit performance of the high-gain laser medium in providing pulses of laser light including the maximum pulse rate of the laser beam and the intensity with which the optical pumping must be performed. Such inherent problems may present significant problems for applications which require high pulse rates and suffer from limited power budgets.
Another problem that is introduced by the utilization of flashlamps to provide optical pumping is the broad spectral width of flashlamp produced light may prove very inefficient in that a large proportion of the light produced by the flashlamp does not serve to generate the inverted population of energy states. Flashlamp light outside of the spectral density range required for generating the inverted population is simply lost into the high-gain laser medium in the form of thermal heating. This heating of the high-gain laser medium may itself produce undesirable effects including beam pointing errors and self-focusing. The heating of the high-gain laser medium may increase to such levels that fracture of the solid-state crystals will limit the maximum peak or average power.
The pulse rate at which the laser amplifier may be switched is also limited by the physical properties of the flashlamps which provide the optical pumping. The electrical switching of the flashlamps is often associated with the thermal heating problems associated with the flashlamps themselves. This upper limit of pulse rate may also be determined in part by the intensity level at which the flashlamps must operate to generate an inverted energy state population above threshold. For example, if the energy transition of interest is near the periphery of the spectral density of the flashlamp, the flashlamp may necessitate operation at a very high power level to generate the inverted population. Such a situation may at the very least limit the duty cycle of the pulse rate to avoid overheating of the flashlamps themselves.
Additionally, the flashlamps intrinsically possess a start up time constant before they begin optical pumping. They do not respond instantaneously with the vertical transition of the electric signal which drives them. Consequently, the maximum pulse rate of the optical amplifier may be limited by the time constant corresponding to the start up of the flashlamps. Another consequence of the intrinsic response limitations of the flashlamps is a lower limit on the width of the pulse which may be generated using such a laser amplification system. Such a problem stems from the similar characteristic of the flashlamps in that they are limited in the speed with which they may switch on and off. The minimum pulse width which may be generated is often dictated by the minimum time in which the flashlamps may turn on and then turn off, including considering of the start up time constant of the flashlamps and evanescent decay of radiation from the flashlamps when turned off.
The present invention overcomes or eliminates the problems and limitations of known systems and methods for detecting high-speed laser-induced ultrasound to provide a system and method for laser beam amplification from a solid state laser that yields high pulse rates, improved pointing stability, and optionally variable pulse rates for non-destructive laser ultrasonic testing of materials, as well as a variety of other uses.
According to one aspect of the present invention, there is provided a method for generating an amplified laser beam at a high pulse rate that includes generating a low-amplification seed laser light signal. The method further includes transmitting the lowamplification seed laser light signal to an amplification component. The low-amplification seed laser light signal is amplified in the amplification component by stimulating emissions of the population inversion that a pumping diode provides. The result of this amplifying step is to yield an amplified laser light signal. The amplified laser light signal is then directed to an output destination.
The present invention provides a system and method for providing amplification of laser light from a solid state laser while maintaining the physical properties of the laser light by minimizing amplification induced distortion. A seed laser possessing desired physical properties including a single longitudinal mode with a desired linewidth is passed through a high-gain laser medium. The high-gain laser medium is optically pumped using a pumping array of laser diodes distributed across the high-gain laser medium. The electric current which drives the pumping array may be a timevarying signal which consequently provides time-varying optical gain and lasing conditions within the laser medium. The amplified laser beam may then be pulsed at a pulse rate corresponding to the frequency of the time-varying signal comprising the electric current which drives the pumping array of laser diodes.
The present invention may be employed in applications which require a particularly narrow or pure spectral density such as applications involving optical interferometry which often require a single longitudinal mode having a very stable center frequency and linewidth. For such applications in which the purity of the initial seed laser is amplified to generate a pulse stream of laser light having a desired intensity level, duty cycle and pulse rate, an optical isolation assembly may be used to minimize back reflection of the laser light into the seed laser light source. Undesirable parasitic feedback may corrupt the seed laser source resulting in deleterious performance of the seed laser including amplitude noise and multimode operation. Such effects may be disastrous for those applications requiring a spectrally pure laser source. One conventional method, well known to those skilled in the art, for providing optical isolation of a laser beam which minimizes back reflections into the original light source comprises a Faraday rotator and two polarizers which provide for light propagation in only one direction through the optical isolation assembly by using the non-reciprocal rotation of a polarized lightwave provided by a Faraday rotator.
Additional optical isolation assemblies may be included in the invention for providing beam splitting of the original laser beam for directing the amplified, pulsed output laser beam or directing of the original laser beam through the high-gain laser medium multiple times for even greater amplification than a single pass through the high-gain laser medium.
The present invention generates an amplified and pulsed laser beam which possesses similar physical properties of the original seed laser including a desired center frequency and linewidth for use within an optical interferometer to perform ultrasonic detection. Additionally, the output intensity of the pulsed, amplified laser beam may be modulated to extend the dynamic range of detection within an optical interferometric system. Electro-optic modulators comprising Pockels or Kerr effect crystals are well known to those skilled in the art for providing intensity modulation of a laser beam.
The present invention provides an important technical advantage by performing optical pumping of a high-gain laser medium using a pumping array of laser diodes. This method permits optical pumping within a very narrow wavelength regime selected by the use of appropriate laser diodes to minimize optically-induced thermal heating of the laser medium. This advantage is provided primarily by the fact that the appropriate choice of laser diodes which comprise the pumping array may be chosen to optically pump within a specific wavelength regime thereby not incurring significant thermal heating by radiation bombardment of the highgain laser medium with optical pumping outside of the energy transition level of interest.
The present invention provides another technical advantage by providing for maximizing the pulse rate and controlling the pulse width of the resulting amplified pulsed laser beam by performing the optical pumping using a pumping array comprising a multiplicity of laser diodes. The maximum pulse rate for the present invention will be determined largely by the speed at which the laser diodes may be turned on and turned off and the allowed duty cycle.
The present invention provides another technical advantage by minimizing amplification induced distortion of a seed laser beam. Many applications, including optical interferometry, require a highly amplified beam with a uniform wavefront and good pointing stability.
The present invention provides another technical advantage by providing for intensity modulation of the output laser beam thereby expanding the detection dynamic range within interferometric systems.